WorldWide Telescope is a free Microsoft Research application and online resource that uses high resolution space imagery to turn your PC into a virtual telescope.
You can start by clicking and dragging on the night sky, using the mouse scroll button to zoom in on any planet, moon or star that appeals. There's a great deal of empty space between the interesting bits, but fortunately Microsoft have also provided some shortcuts at the bottom of the screen. By default this is set to the solar system, and so a quick double-click will have you admiring Earth, wondering if there really is life on Mars, or marvelling at the beautiful rings of Saturn.
This is just the start, though. If you want to take a much closer look at Mars, say, then choose the Panorama option and pick one of the Pathfinder options. This will display a 360 degree panorama shot of the desolate Martian landscape, that you can rotate with your mouse. No matter how many NASA pictures you've seen, this somehow provides a much more immersive experience, a far clearer way of understanding what the planet is really like. And there are more than 50 of these panoramas to explore.
When you're tired of the solar system then it's easy to head into deep space, where you can view gorgeous photos from Hubble, the Spitzer Space Telescope, the Chandra X-ray Observatory, and many other sources. And perhaps the best feature is the guided tours, which zoom you around galaxies, nebulae, dark holes and more, while top astronomers deliver clear and accessible narrations describing exactly what you're seeing.
A fascinating, fun and (gently) educational way to explore the universe